Re: [PATCH v1 3/3] rust: sync: atomic: Add i8/i16 xchg and cmpxchg support

[Boqun Feng <boqun.feng@gmail.com>]

On Sun, Dec 28, 2025 at 09:05:46PM +0900, FUJITA Tomonori wrote:
> Add atomic xchg and cmpxchg operation support for i8 and i16 types
> with tests.
> 

I think we also needs the following, otherwise architectures may
accidentally enable Rust but don't have the correct atomic
implementation for i8 and i16.

diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
index 248d26555ccf..a4e5bbd45eb2 100644
--- a/rust/kernel/sync/atomic/predefine.rs
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -5,14 +5,22 @@
 use crate::static_assert;
 use core::mem::{align_of, size_of};

+// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
+// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
+//
 // SAFETY: `i8` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
+#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
 unsafe impl super::AtomicType for i8 {
     type Repr = i8;
 }

+// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
+// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
+//
 // SAFETY: `i16` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
+#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
 unsafe impl super::AtomicType for i16 {
     type Repr = i16;
 }

I can fold it into your patch if that works.

Regards,
Boqun

> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/sync/atomic/internal.rs  | 2 +-
>  rust/kernel/sync/atomic/predefine.rs | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
> index 1b2a7933bc14..55a80e22a7a0 100644
> --- a/rust/kernel/sync/atomic/internal.rs
> +++ b/rust/kernel/sync/atomic/internal.rs
> @@ -274,7 +274,7 @@ fn set[release](a: &AtomicRepr<Self>, v: Self) {
>  );
>  
>  declare_and_impl_atomic_methods!(
> -    [ i32 => atomic, i64 => atomic64 ]
> +    [ i8 => atomic_i8, i16 => atomic_i16, i32 => atomic, i64 => atomic64 ]
>      /// Exchange and compare-and-exchange atomic operations
>      pub trait AtomicExchangeOps {
>          /// Atomic exchange.
> diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
> index 51e9df0cf56e..248d26555ccf 100644
> --- a/rust/kernel/sync/atomic/predefine.rs
> +++ b/rust/kernel/sync/atomic/predefine.rs
> @@ -149,7 +149,7 @@ fn atomic_acquire_release_tests() {
>  
>      #[test]
>      fn atomic_xchg_tests() {
> -        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
> +        for_each_type!(42 in [i8, i16, i32, i64, u32, u64, isize, usize] |v| {
>              let x = Atomic::new(v);
>  
>              let old = v;
> @@ -162,7 +162,7 @@ fn atomic_xchg_tests() {
>  
>      #[test]
>      fn atomic_cmpxchg_tests() {
> -        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
> +        for_each_type!(42 in [i8, i16, i32, i64, u32, u64, isize, usize] |v| {
>              let x = Atomic::new(v);
>  
>              let old = v;
> -- 
> 2.43.0
>